Conventionally, a vinyl acetate polymer, a chloroprene polymer, an acrylic ester polymer, natural rubber, an urethane polymer and the like have been used as a polymer for adhesives. In particular, a chloroprene polymer has been suitably used for adhesives such as solvent type contact adhesives and graft adhesives since high bond strength can be obtained at low compression bonding for a wide variety of adherends. However, a regulation on volatile organic compound emissions and a solvent regulation have been called for years in consideration of the recent environmental pollution and the health of human beings. Aqueous adhesives using a chloroprene latex has been increasingly developed in order to meet these needs.
However, because of the strong desire for the bond performance, a problem of the aqueous adhesive has been pointed out regarding the bond strength lower than that of a conventional solvent type. Consequently, in order to meet the ever more sophisticated needs to improve the bond strength, particularly the initial bond strength, an adhesive having high crystallization rate and showing excellent initial bond strength in a short time is being developed. However, the initial tack in a bonding step at room temperature rather than heat resistance is more important for the product to be used (in service) at a relatively low temperature such as 60° C. or lower, for example, for the use of bonding cushions, fabrics and cloths in the field of furniture and construction materials. That is, despite the importance of the initial tack with slight contact pressure, the adhesive using a chloroprene latex has not adequately fulfilled the expectations. At the same time, water-resistant property is important as well.
In particular, in the bonding of the adherends (substrates) such as foam, the contact adhesion is extremely important because it is necessary to attain the initial bond strength in a short time with slight contact pressure such that the substrate would not be damaged (deformed/destroyed). A technique of using a tackifier agent with a low-melting point may be employed so as to increase the initial tack, however, in this case, a problem of the lowered adhesion arises due to the cohesion peculiar to a chloroprene polymer. Also, though a spray coating is often employed to facilitate and to save labor of applying an adhesive, the use of the compounding agent in a large amount tends to cause nozzle clogging of the spray due to the precipitation of the solid compounding agent, which leads to a processing problem. For these reasons, there is a growing need of the adhesive for being excellent in the initial tack (contact adhesion) and water-resistant property while maintaining the heat-resistance and the bond strength by the chloroprene latex itself.
The conventional chloroprene polymer latex has had a problem that it failed to achieve a good balance among excellent initial bond strength, initial tack and water resistance.
The examples of the prior art relating to the chloroprene polymer latex include JP-A-H08-504228 (U.S. Pat. No. 5,332,771). However, the latex cannot be expected to attain high initial bond strength because it is a copolymer of chloroprene and 2,3-dichlorobutadiene of at least 2 mass % and therefore cannot attain a desired cohesion due to a low crystallization rate. JP-A-H09-31429 (U.S. Pat. No. 5,977,222) as a second example does not have a description of the crystallization of the chloroprene latex or the polymerization temperature and does not necessarily attain the high initial bond strength. In the case of JP-A-H11-158327 (U.S. Pat. 6,525,132) as a third example, the invention defines that the content of the gel (crosslinking component) in the polymer is 3 to 40 mass %, and therefore, while the sufficient heat resistance is attained, it is difficult to say that the sufficient initial tack and the water-resistant property are achieved due to an excessive amount of the gel. or the polymerization temperature and does not necessarily attain the high initial bond strength. In the case of JP-A-H11-158327 (U.S. Pat. No. 6,525,132) as a third example, the invention defines that the content of the gel (crosslinking component) in the polymer is 3 to 40 mass %, and therefore, while the sufficient heat resistance is attained, it is difficult to say that the sufficient initial tack and the water-resistant property are achieved due to an excessive amount of the gel.
The conventional technology relating to the above-mentioned chloroprene polymer latex has a problem in achieving a balance among the initial bond strength, contact adhesion and water resistance, while a high-level heat resistance: e.g. the resistance at 80° C. or higher is ensured. That is, the conventional chloroprene polymer latex has a problem of the insufficient initial tack (contact adhesion) for use in an aqueous adhesive.    Patent Document 1: JP-A-H08-504228    Patent Document 2: JP-A-H09-31429    Patent Document 3: JP-A-H11-158327